Light clock promises finer time

By
Eric Smalley,
Technology Research NewsMaking
the most of time requires that time be well defined. And in order to be
precise, a definition of time must involve something that happens very quickly.

The current definition of a second is the duration of 9,192,631,770
oscillations of cesium atoms excited by microwaves. Today's cesium atomic
clocks are accurate to within one million billionth of a second or 1 second
in 30 million years.

This is precise enough for a cluster of orbiting satellites to calculate
the position of a stationary object to within a millimeter. For moving objects
like cars and planes, however, the accuracy is a few meters, which is not
enough to allow a global positioning system to, for instance, automatically
land a plane.

Researchers from the National Physical Laboratory in England have
made a prototype atomic clock that divides time into slices based on optical
radiation, or lightwaves, rather than microwave radiation. Such clocks could
eventually improve global positioning systems, make space exploration more
accurate, and more accurately test the laws of physics, said Helen Margolis,
a principal research scientist at the National Physical Laboratory.

Because lightwaves are smaller and faster than microwaves, optical
clocks have operating frequencies as many as 100,000 times higher than today's
cesium microwave clocks, said Margolis. "They divide time into much finer
slices, and therefore have the potential to give much higher accuracy,"
she said.

Today's atomic clocks measure the vibration frequency of cesium
atoms to calibrate quartz crystal electronic oscillators. A laser excites
the atoms to the energy level where they resonate with a microwave field
that is tuned by an electronic oscillator. The microwave field cycles through
a range of frequencies close to the cesium atoms' resonant frequency, and
as the microwaves resonate with the atoms the atoms give off energy in the
form of photons. A photodetector measures the peak amount of light and locks
the microwave field on that frequency.

Optical atomic clocks use lasers instead of microwaves to resonate
the atoms, and atoms that have higher resonant frequencies than cesium.
The researchers' prototype uses a single strontium ion that is held in an
electromagnetic trap and laser cooled to near absolute zero, said Margolis.
Another laser causes the ion to oscillate at its resonant frequency -- 444,779,044,095,484.6
cycles per second.

The difference between the strontium frequency and the cesium frequency
is the difference between 1 second and 13 and a half hours. This higher
frequency could lead to optical atomic clocks that are so accurate they
would lose less than a second over the lifetime of the universe.

One challenge is that electronic photodetectors are too slow to
measure such high frequencies. Researchers have recently developed another
tool that is fast enough, the femtosecond laser frequency comb. Frequency
combs are lasers that emit a pulse every million billionth of a second,
or femtosecond. The light emitted by the lasers covers the wavelengths of
the spectrum of visible light in discrete, narrowly-spaced intervals equal
to the frequency of the pulses. "This acts like an optical frequency ruler...
with the frequency of every tooth of the comb known precisely," said Margolis.

Researchers measure optical atomic clocks by matching their output
to a frequency on a frequency comb.

The researchers' strontium ion prototype is accurate to 3.4 million
billionths of a second, which is three times more accurate than a prototype
optical atomic clock based on a single mercury ion demonstrated by U.S.
National Institute for Standards and Technology scientists but about three
times less accurate than the best cesium clocks, said Margolis.

The strontium ion clock is potentially precise enough that it would
be limited by the current definition of the second, said Margolis. The frequency
combs are calibrated by cesium clocks. Given a redefined second, optical
clocks could be considerably more accurate, she said. "We believe that
future generations of such optical clocks could be nearly a thousand times
more accurate than the best clocks of today," she said.

Such a clock would not lose a second over the lifetime of the universe.

Optical clocks based on a more precise definition of the second
would improve global positioning systems, and are crucial to deep space
exploration, said Margolis. "To send a spacecraft millions of kilometers
into an unknown part of the universe -- and perhaps ask it to land gently
in a particular place -- will require extremely accurate clocks to synchronize
its navigation equipment."

More accurate time measurement is also useful in testing the laws
of physics, said Margolis. "Optical clocks will also provide a powerful
tool... to explore questions such as 'are the fundamental physical constants
really constant or do they change with time'."

The researchers' optical clock can be used now for fundamental science
applications like testing the consistency of physical constants, said Margolis.
It could be used for global positioning system ground stations in five years
and on satellites in 10 years, she said.

Margolis's research colleagues were Geoffrey Barwood, Guilong Huang,
Hugh Klein, Stephen Lea, Krzystof Szymaniec and Patrick Gill. The work appeared
in the November 18, 2004 issue of Science. The research was funded
by the UK Department of Trade and Industry.